Higher sulfur oxides are a group of chemical compounds with the formula SO3+x where x lies between 0 and 1.
They contain peroxo (O−O) groups, and the oxidation state of sulfur is +6 as in SO3.
Monomeric SO4 can be isolated at low temperatures (below 78 K) following the reaction of SO3 and atomic oxygen or photolysis of SO3–ozone mixtures.
The favoured structure is: Colourless polymeric condensates are formed in the reaction of gaseous SO3 or SO2 with O2 in a silent electric discharge.
The structure of the polymers is based on β-SO3 (one of the three forms of solid SO3) with oxide bridges (−O−) replaced randomly by peroxide bridges (−O−O−).